Japanese Laid-Open Patent Publication 8-213259 describes a conventional ignition coil that is inserted into a plug hole of an automobile engine. Referring to FIG. 10 hereof, an open magnetic circuit is formed by primary coil 3 and secondary coil 5 disposed around rod-shaped magnetic core 1. This provides a structure which is compact and that has a small diameter. Also, magnetic leakage is prevented by disposing outer cylinder 9 around the external surface of transformer 7.
Furthermore, in this ignition coil, plate-shaped magnet 11 is disposed at one end or both ends of magnetic core 1 to provide reverse bias for magnetic field B1 generated by primary coil 3. The magnetic flux density in magnetic core 1, which has been magnetized by primary coil 3, is decreased by magnet 11. The coercive force from magnet 11 serves to decrease the residual field in magnetic core 1 brought on by magnetic field B1. When a direct current voltage is intermittently applied to primary coil 3, the changes in the flux density in magnetic core 1 are increased, thus providing more efficient energy retrieval at secondary coil 5.
In the ignition coil described above, outer cylinder 9 supplements magnetic core 1. However, because the magnetic circuit between magnetic core 1 and outer cylinder 9 is interrupted, the actual magnetic leakage is high. This prevents the efficient use of magnetic field B1 generated at primary coil 3 and makes the retrieval of energy less efficient.
Magnetic field B1 is generated at an end of magnetic core 1 by primary coil 3. A large proportion of the field from the end of magnetic core 1 to the end of outer cylinder 9 extends along the direction perpendicular to the axis of magnetic core 1, as shown in FIG. 10. In the ignition coil described above, magnetic field A1 generated by magnet member 11 is formed along the thickness of magnet 11, i.e. along the axis of magnetic core 1. Thus, magnetic field B1, generated by primary coil 3 between magnetic core 1 and outer cylinder 9, is not weakened by magnet 11. Instead, magnetic field B1 is formed between magnetic core 1 and outer cylinder 9 in such a matter that it is diverted around magnet 11. As a result, reverse-bias magnetic field A1 from magnet 11 cannot act efficiently against magnetic field B1 of primary coil 3. This also prevents the secondary output from increasing. FIG. 11 shows composite magnetic field C1 formed by magnetic field B1 generated by primary coil 3 and magnetic field A1 from magnet 11. In the figure, composite magnetic field C1 avoids magnet 11 without being weakened.
In Japanese Laid-Open Patent Publication 3-154311, there is described an ignition coil which uses a toroidal permanent magnet for reverse-biasing. In this ignition coil, a ring-shaped core is inserted between a rod-shaped magnetic core and a surrounding outer cylinder. A closed magnetic circuit is formed by the outer cylinder and the ring-shaped core. The toroidal magnet is inserted in the magnetic gap formed between the magnetic core and the ring-shaped core, thus providing a structure with a closed magnetic circuit. The idea behind this technology is different from that of the present Invention. Instead of using a ring-shaped core, the present Invention involves a reverse-biasing toroidal magnet inserted directly between the magnetic core and the outer cylinder, thus providing a main magnetic circuit that is open.
Also, Japanese Laid-Open Patent Publication 3-154311 does not disclose the direction of the magnetic field generated by the reverse-biasing magnet, and it is unclear how the reverse-biasing magnetic field is applied. Even if the magnet of this Publication generates magnetic field A.sub.1 along the thickness of magnet member 11 (i.e. along the axis of the magnet core 8), as shown in FIG. 10, it would not be possible to obtain an appropriate reverse-biasing magnetic field as shown in FIG. 11.
Thus, magnetic field B1, which is formed by primary coil 3 between magnetic core 1 and outer cylinder 9, is not weakened by magnet 11. Instead, magnetic field B1 is formed between magnetic core 1 and outer cylinder 9 so that it is diverted around magnet 11. Thus, reverse-bias magnetic field A1 formed by magnet 11 cannot be applied effectively against magnetic field B1 of primary coil 3. This presents a problem when trying to increase the secondary output. FIG. 11 shows composite magnetic field C1 formed by magnetic field B1 (generated by primary coil 3) and magnetic field A1, generated by magnet 11. The figure shows how the path of composite magnetic field C1 is diverted around magnet 11 without being weakened.
Even if the magnet member of this publication were to generate a magnetic field along the radial direction of the magnet as in the present Invention, this ignition coil uses a closed magnetic circuit where a magnet is inserted into the magnetic gap in the narrow space between the ring-shaped core and the magnetic core. Thus, the inserted magnet member would be small and the biasing would be inadequate.
In order to overcome the problem described above, the object of the present Invention is to provide an ignition coil that uses the magnetic field generated by the primary coil effectively; applies a reverse-biasing magnetic field to the magnetic field generated by the primary coil; and allows the reverse-biasing magnet member to be easily and reliably positioned and fixed. This also makes it possible to provide a more compact ignition coil with a smaller diameter and greater efficiency in energy retrieval.